The present invention relates generally to turbines having a plurality of circumferentially-spaced buckets about the periphery of a rotor wheel, and particularly, to bucket damper pins disposed between adjacent buckets for damping bucket vibrations.
As is well known, turbines generally include a rotor comprised of a plurality of rotor wheels, each of which mounts a plurality of circumferentially-spaced buckets. The buckets each typically include an airfoil, a platform, a shank and a dovetail, the dovetail being received in a mating dovetail slot in the turbine wheel. The airfoils project into the hot gas path of the turbine and convert kinetic energy into rotational mechanical energy. During engine operation, vibrations are introduced into the turbine buckets and if not dissipated, can cause premature failure of the buckets.
Many different forms of vibration dampers have been proposed to minimize or eliminate vibrations. See, for example, U.S. Pat. Nos. 6,851,932; 6,354,803; 6,390,775; 6,450,769; 5,827,047 and 5,156,528.
The '932 patent describes a damper pin located between each adjacent pair of buckets for reducing the amplitude of vibratory stresses at full speed—full load and full speed—no load conditions.
Nevertheless, today's high-firing-temperature gas turbines require improvement in corrosion and oxidation resistance capabilities for bucket damper pins exposed to a high temperature environment, while maintaining required sealing, damping and wear characteristics. Damper pin corrosion and oxidation distress can cause loss of damping leading to mechanical failure, liberation of the bucket causing damage to other turbine components, and/or compressor discharge flow leakage leading to reduced engine efficiency, etc.
Older damper pin designs have not required corrosion and oxidation protection since the damper pins were used in gas turbines operating at lower firing temperatures, and since film cooling carryover from upstream nozzle side walls tended to reduce the temperature of the air to which the pins were exposed. New gas turbine designs with closed loop airfoil cooling, however, significantly reduce film cooling of upstream airfoils in an attempt to increase turbine efficiencies. The reduction in film cooling, along with the increase in firing temperatures, significantly increase the temperature at the leading edge of the damper pins. In addition, in previous designs, increased wheel space purge flow was required to maintain the required temperature to assure the damper pins did not oxidize. The addition of purge flow, however, reduces turbine efficiency, and thus is not an acceptable solution.